Centrifugal casting



A. EjscHuH ET AL 2,399,606

CENTRIFUGAL CAS TING April 30, 1946.,`

2 Sheets-Sheet l Filed Deo. 4, 1943 N M M,

(YY 'n WITNESS April 30, 1946. A. E SCHUH ET AL 2,399,606

CENTRIFUGAL CASTING Filed Dec. 4, 1943 2 Sheets-Sheet 2 PHOTOGRAPH ofSURFQCE 0F Conn/v6,

MnGN/F/a fa fnv/65,

Fl-Lig, 7.

PHTOGRHPH OF C/POSS SECT/0N 0F CO/T//VGJMHGN/F/E ZO TMES.

/NVENTRS w/T/vsss er HTTOR/YEY Patented Apr. 3o, 194e CENTBIFUGALCASTING Arthur E. Schuh, Delanco, and Alfred Boyles, Burlington, N. J.,asslgnors to United States Pipe and Foundry Company, a corporation ofNew Jersey Application December 4, 194s, serial No. 512,958

17 Claims.

'This invention relates to the manufacture of tubular metal castings,and is directed particularly to certain improvements in the manufactureof such castings centrifugally in permanent metal molds rotatedhorizontally or substantially In centrilugally casting in this mannerand particularly when casting a relatively long tubularbody, that is,whose length exceeds three or four times its outside diameter, the arthas long been confronted Withthe problem of producing a casting in whichthe metal is completely fused throughout the wall `and which has asubstantially-defect-free outer surface.

While this problem is more or less acute according to the particularmethod of centrifugal casting employed, it has not'heretfore beensolved.

For instance, one particular method of centrifugal casting heretoforeemployed in the production of long tubular bodies in permanent metalmolds involves the pouring of the charge of molten metal into the metalmold in such a manner that the distribution of the metal longitudinallyof the mold is effected primarily by the action of centrifugal force. Acommon Way of doing this is to pour the metal from a spout occupying arelatively fixed position lengthwise l of the mold and located say atone end of. the

mold. However. attempts to produce long tubular castings centrifugallyby such a method have heretofore resultedV in the formation of castingshaving so-called "pinholes and laps or cold shu which render thecastings commercially unacceptable.

In another method which is widely employed commercially the molten metalis delivered to the mold by means of a pouring trough relativelyretractable over the length of the mold so that the molten metal isdeposited progressively to )form a helix, the convolutions. of whichfuse together to form an integral structure. In this method the metal isdistributed longitudinally over the mold surface primarily by means ofthe retractive pour, and the centrifugal torce holds the molten metalagainst the mold, as in all centrifugal casting. However, in actualpractice the metal upon being so deposited does spread to. some Aextentbecause of the lateral pressure resulting from the centrifugal force,and is thus caused to ow longitudinally of the mold to` advance beyondthe helical zone of deposition. This spreading is sometimes excessive,and is frequently irregular, or, as it is sometimes called,discontinuous, on the leading edge, and then results in the formation ofexcessively thin oi`1` shoots which solidify almost instantaneously andproduce small laps or cold shuts in the outer surface or portion of thecasting. While this 5 method reduces the troublesome defects. it doesnot solve the problem above referred to.

As a result of extensive efforts to make long tubular casting free ofsuch defects, centrifugally from steel or other metals or alloys pouredinto horizontal or substantially horizontal permanent metal molds .byeither of these methods, it has been found that the incomplete fusion ofthe metal and the defects on the outer4 surface of the casting are dueto' an uncontrolled advance of the charge of molten metal lengthwise ofthe mold without sullciently partaking of the rotational motion of themold. When the advancing molten metal slips over the mold, movinglengthwise thereof intermittently and/or too rapidly, it tends to resultin the formation of pinholes originating at the outer surface of thecasting. The laps or cold shuts are produced when the leading `vedge ofthe molten metal moves longitudinally of the mold more or lessintermittently and/or discontinuously, 'at a speed wherebyit becomes sothin that it rapidly cools, prematurely solidifies and also oxidizes, tosuch an extent that it does not properly Weld to or unite with the massof metal which subsequently flows '301over it as the distribution ofthecharge progresses. These pinholes and laps or cold shuts sometimesextend throughout the thickness of the wall of the casting, but usuallyoccur only in the outer portion of the wall or in what is often termedthe outer surface.

' The present invention hasfor its principal object to provide aprocedure whereby long tubular castings made of 'steel or other metalsor alloys may be centrifugally cast in horizontal or ap- 40 proximat-elyhorizontal permanent metal molds in such a manner that the castings havethe metal completely fused throughout their walls and have their outerportions substantially free from pinholes and laps or cold shuts, aswould detract from the quality of the castings or render themcommercially unacceptable.

.More particularly, an important object of the invention is to solve theproblem above referred tol by employing, in a centrifugal casting methodwherein the molten -metal is poured into the mold while the mold isrotated to distribute the metal both circumferentially andlongitudinally therein, a procedure in which the pick-up of the moltenmetal for rotation with the mold is accelerated, the flow of-the metallongitudinally of the mold is retarded, and, while maintaining theover-all rate of solidification of the casting substantially the same asin the bare metal mold, the heat flow to the mold from the leading edgeof the longitudinally advancing molten metal is momentarily arrested tomaintain the leading edge at-- a temperature to fuse with the moltenmetal which subsequently lflows thereover.

Another object of the invention is to effect the procedure abovereferred to, by providing a relatively thin refractory mold coating ofsuch a character and texture that it both serves as a thermal barrier toarrest and controlthe rate of solidiiication of the leading edge of themolten metal, and provides a surface of controlled roughness to give thedegree of traction required between the molten metal and the mold. Thethermal barrier action is necessary in order to assure complete fusionof the metal forming the outer portion of the casting as the moltenmetal is propagated in the mold, but requires no more than a momentaryarrest of the heat flow to the mold without substantially slowing downthe over-all heat flow and the quick freezing of the metalcharacteristic of casting in metal molds or retarding the rate ofcasting production. Hence thesuperior grain structure of the cast metaland other well-known advantages attained by and inherent in castingcentrifugally in a metal mold are retained. The traction is required toaccelerate the speed with which the poured charge of the molten metalpartakes of the rotary movement of the mold and give a quick pick-up,and also to control the longitudinal propagation of the metal in themold, that is, to keep the molten metal from slipping over the mold in alongitudinal direction intermittently and too rapidly and with adiscontinuous and excessively thin leading edge. These properties andfunctions of the coating act conjointly to produce the desired result.

A further object is to provide such a coating which is applied byspraying a liquid suspension of the coating material on the mold, and inwhich the above mentioned required properties thereof may be varied andcontrolled by the regulation of the spraying operation.

A composition which has been found suitable for providing a coatinghaving the desired characteri-stics above referred to is composed of asuspension of silica iiour and bentonite in water, and is applied to themold by a spraying operation in which the composition is atomized anddischarged on the inner surface of the heated metal mold by a relativereciprocation of a spray nozzle thereover. While the coating compositionmay be varied, a uniform aqueous suspension made up in the proportionsof 1 litre of water, from 500 to 1000 grams of silica our and from l to40 grams of bentonite has proved to be satisfactory. The particles ofsilica flour should be of such a fineness that the composition issprayable in an atomizer type of spray, and are preferably of about #325sieve size or finer. Although bentonite aids materially in keeping thesilica flour in suspension in the water, it is desirable to agitato theliquid composition more or less continuously in order that a uniformsuspension may be maintained during the application-of the composition.

In applying such a composition to the inner surface of the mold it isessential that the mold be heated to a temperature within the range o`f250 to 800 F., with a preferred range of 350 to 500 F. The compositionshould be atomzed and sprayed on the mold at such a rate that thecarrier liquid will evaporate very quickly after the sprayed globulescome into contact with the heated surface. The mold should not be sohot, however, as to vaporize the liquid before the globules aredeposited. When the composition is applied in the manner indicated, thesolid refractory and binder materials of each globule remain essentiallyin the position where they are deposited on t e mold surface with theresult that an increm ntal coating is formed by the spraying operation.As the coating is thus formed by an accretionary growth of the solidmaterial of these globules and the spray conditions are such that theglobules do not spread or run together on the mold surface, the coatingwhich is thus built up is provided with a surface of controlledroughness, which is formed of a multiplicity of projections orprotuberances.

The particular shape and contour of the protuberances do not appear tobe important, although a surface which, upon being enlargedphotographically, resembles somewhat the surface of a cauliflower hasbeen found to produce very satisfactory results. The protuberances onsuch a surface are, of course, irregular in shape and size and areusually so arranged as to be noticeably spaced from each other. It isnot to be understood, however, that such irregularity is essential. Itis desirable that the coating be so formed that the protuberances mergeinto a basal portion or that they be basi-connected in order that thesurface of the mold may be substantially covered by the coating, and inorder that the charge of molten metal may be subjected substantiallyuniformly to the thermal barrier action of the coating.

The character of the coating surface may be readily varied to avoid onewhich is too smooth to effect the desired control over the distributionof the molten metal in the mold or one which is rougher than need be forthe purpose. Desired variations in the roughness of the surface of thecoating may be made by suitably controlling various features of theprocedure which is followed in applying the coating composition, such asby altering the shape and direction of the spray jet issuing from thespray nozzle, a1- tering the distance of the nozzle from the surface ofthe mold, changing the degree of atomization of the coating compositionor varying the air pressure employed, varying the amount of coatingcomposition applied to a given area of the mold surface in a given timeinterval, and varying the extent of the incremental formation of thecoating.

While different forms of nozzles or spray heads may be usedin applyingthe coating composition,

a fan spray has been found suitable as it can be readily controlled tovary the dimensions and directions of the fan or sheet emitted by thenozzle. In coating a rotating cylindrical mold by relative reciprocationof a spray device longitudinally within the bore of the mold, the spraydevice is preferably so arranged that the fan or sheet of spray extendsparallel to the longitudinal axis of f the mold. Such an arrangement isdesirable in order to produce a coating having a rough surface respectto the mold surface may 'also be vused to effect changes in thecharacter of the surface of.

.the coating. Owing to the greater concentration of the spray upon agiven area of the mold surface, 4a smooth surfaced coating may be madewhen the nozzle is directed at an angle of 90 with respect to the moldsurface, while a rough surface may be provided on the coating with thenozzle directed at an angle of 45. It has also been found that anincrease in the air pressure utilized with an air spray device tends toproduce a rougher coating, but this tendency may be neutralized if thespray knozzle is operated too close to the mold surface.. Increasing thedistance of the nozzle from the mold surface tends to increase `theroughness of the surface of the coating.

'I'he roughness of the coating surface may also be varied by alteringthe quantity of the coating composition lsprayed upon a given area f themold. surface in a given time interval. When applied at a suitable rate,the solid materials of the separate globules remain in place so that theaccretionary growth produces a rough surfaced coating. Upon increasingthe amount of the composition applied tethe mold, or the rate ofapplication of the composition, the composition tends to spread over themold surface by reason of the slower evaporation of the carrier liquid,with the result that a coating with a smoother surface is formed. V

When other conditions are such as to produce a coating with a roughsurface of suitable character, the roughness of -the lsurface tends toincrease as the coating is incrementally-built up by the sprayingoperation. This is believed to be due to the fact that the separatemasses of solid material in the first globules which strike the moldsurface project from the surface and are thus in position to interceptmore of the sprayed material than the surrounding mold surface. Thesesmall masses of solidi material thus grow more rapidly than thesurrounding areas of the mold surface, and this rapid growth continuesuntil the thickness of the protuberances thus formed becomes suicient toinsulate their exposed surfaces from 45 the heat effect of the mold andthus prevent the liquid from evaporating quickly from theV additionalglobules or parts of globules which are directed upon them. When thisoccurs there is ultimately attained a condition whereby the sprayedglobules now together into a relatively smooth surface which is nolonger suitable for the production of defect-free long centrifugalcastings in accordance with this invention.

It has been found that the average thickness of a coating of thiscomposition may vary within a range of from 0.002 inch to 0.08 inch;that the average number of protuberances may vary from to 120 per linealinch measured in any direction across the surface of the coating. andthat the average height of'the protuberances above the surrounding areaof the coating may vary Within a range of from 0.001 inch to 0.050 inch.As to the roughness of the surface of the coating,

to put it another way, it is ccomparable to and may vary within therange of No, 24 to No. 240 abrasive the particular method of centrifugalcasting wherein action of centrifugal force is primarily relied upon todistribute the molten metal longitudinally of the mold, the followingranges may 5 be used: an average thickness within approximately from0.01 `inch to 0.08 inch; 10 to 50 protuberances per inch when measuredin any direction acrossl the surface of the coating, 20 to 25protuberances per inch being preferred; and an l0 averagev protuberance`height ranging from approximating 0.005 inch to 0.050 inch, theroughness being of the order and within the range of No. 24 to No. 100according to the abrasive cloth or paper scale and preferably within therange of No. 40 to No. 60. On the other hand, in the particular form ofcentrifugal casting in which the moltenmetal is delivered by means of apouring trough relatively retractable over the length of the mold, ithas been found that this coating may be thinner and of a lesser order ofroughness and still give satisfactory results.

The above composition, withthe proportions given, produces a coatingwhich adheres together and to the mold and yet which is sufficiently 25friable upon the completion of the casting operation that the particlesdo not adhere to each other and to the mold to such an extent as to'interfere with the ready withdrawal of the casting from the mold. Thiscoating appears to be somewhat more friable after the casting operation`than before it. In this connection we have discovered thatnotwithstanding such Inability, the coating will resist the washing awayeffect of the r molten metal stream as it is poured into the mold, 3"and furthermore the protuberances forming the rough surface ofthecoating will effectively engage with, and control the movement of, themolten metal without beingcrushed thereby. So far as we are aware, it isnovel to employ a coat- .ing of this character for permanent metal moldsused in centrifugal casting. It would not ordinarily be expected thatsuch a coating would act in the manner above indicated, and that it doesso may be due to the fact that the forces exerted on the coating by themolten metal are actually of a lower order than the art has understoodthem to be. It is also to be noted that the friability may be varied andcontrolled by varying the proportionl ofthe bentonite in the coatingcomposition.

A coating of this composition is of temporary character, and is appliedto the mold before each casting operation. After `the application of thecoating, the coated mold may be subjected to handling, inspection,recoating, and/or storage, as required, without interfering with itssubsevquent use. Upon the withdrawal of the casting, the coating may,because of its friability, be readily removed from the mold by means ofa wire brush and/or a jet of compressed air, to facilitate thepreparation of' the mold for the next casting.

Irrespective ofthe other advantages attained through the use of a thincoating of this particular composition, its chief advantage andimportance lies in providing both a controlled, momentary thermalbarrier and an exposed surface of controlled roughness, the roughsurface producing the required traction to cause a quick pick-up of themolten metal to partake of the rotary movement of the mold and to keepthe metal from slipping over the mold in a longitudinal directionintermittently and too rapidly and with a discontinuous and excessivelythin leading edge, and theaction of the thermal barrier providing amomentary arrest of the heat ilow to the metal mold to assure thecomplete fusion of the metal forming such leading edge and the outersurface of the casting withthe oncoming metal subsequently flowingthereover as the metal is propagated in the mold, while maintaining theover-all rate of solidiilcation of the casting substantially the same asin the bare metal mold. By the expression substantially the same as inthe bare metal mold we mean that the over-al1 rate of solidiflca'tion ofthe casting is of that order of `quick freezing which is well-known inthe art as characteristic of centrifugal casting in a metal mold withits attendant advantages, as distinguished fromthe slow freezingcharacteristic of centrifugal casting in a refractory mold with itsattendant disadvantages. The thickness and the order of roughness areboth affected by the accretionary growth of the coating as thecomposition is sprayed. v

Various other compositions may be used in the practice of the inventionto provide, when sprayed upon the mold, a thin refractory coating havingboth the required thermal barrier action and the surface roughnessrequired for traction. For instance, while silica flour is asatisfactory refractory since it is not difficult to obtain and, whenapplied as above described, produces the necessary resistance to heat toprovide the requisite thermal barrier action, any other suitablerefractory materials, such as, for example, the oxides of aluminum,magnesium, beryllium, zirconium, chromium, etc., may be used. Asindicated above, bentonite has been found to be a satisfactory binder asit serves not only as a binding agent for the particles of refractorymaterial in the coating, but as a suspending agent for the particles inthe liquid carrier used in spraying the coating composition on the mold.Nevertheless other suitable binding agents may be used, provided theyhavethe necessary adhesive qualities to enable the coating to withstandthe forces exerted thereon by the charge of molten metal, as well as toadhere properly to the metal mold, and are sufficiently refractory andfree of components tending to emit gases during the casting operation.Furthermore while water is a convenient carrier vehicle for the solidine gredients of the composition it is to be understood that any othersuitable liquid may be employed for this purpose.

Regardless of the particular composition used in the practice of theinvention it is to be noted that the magnitude and distribution of theprotuberances on the inner surface of the coating are such that thetraction of said protuberances on the molten metal serves both toaccelerate the pickup of the molten metal for rotation with the mold andto retard the flow of the molten metal longitudinally of the mold, andthat the thickness of the coating is such that the thermal barrieraction thereof serves to momentarily arrest the heat flow to the moldfrom the longitudinally advancing edge of the molten metal to maintainlsuch leading edge at a temperature to fuse with the molten metalsubsequently fiowing thereover. Theseare essential physicalcharacteristics of the coating to enable it to perform its requiredfunction in carrying out the present invention.

In order that the present improvement may be better understood,reference is made tothe accompanying drawings illustrating apparatus forthe practice of one form of the invention, and a coating suitable foruse in producing tubular casta,soo,ooe f ings in accordance with the.invention. In the drawings,

Fig. 1 is a diagrammatic side elevational view of such apparatus,certain parts thereof being shown in section;

Fig. 2 is a transverse sectionaly view taken on y the line 2-2 'of Fig.1;

Fig. 3 is a sectional, perspective view of the right end of themold-illustrated in Fig. 1, showing the pouring device dischargingmolten metal into the mold, also showing the manner in which the moldcoating operates to control the forward flow of the molten metal in themold; i

Fig. 4 is a diagrammatic plan view, on an enlarged scale, showing thesurface of the improved coating;

Fig. 5 is a diagrammatic sectional view of the coating taken on the line5-5 of Fig. 4;

Fig. 6 is an enlarged photographic view of the surface of a coating madein accordance with the invention; and

Fig. 'I is an enlarged photographic view of a cross section of thecoating shown in Fig, 6.

By way of specific illustration, the invention will now be described andshown in connection with the particular method of centrifugal casting inwhich centrifugal force is primarily depended upon to distribute themetal longitudinally of the mold, or, in other words, in which anon-retractive type of pour is emp1oyed. According to this form of theinvention, it may be conveniently carried out with a cylindrical mold l0which is supported in a horizontal position upon two sets of rollers IIin a manner well understood in the art. The two aligned rollers II oneach side of the mold I0 are secured on a shaft I2 supported in bearingblocks I3 on beams I4. These beams may be laid on a floor I5 serving asa base for the entire apparatus. The projecting end of one shaft II maybe provided with a pulley I6 or other suitable means connected to apower device for driving the shaft and thereby rotating the mold.Annular grooves Il may be provided in the outer surface of the mold toserve as tracks for the rollers II in order that the mold may be heldagainst endwise shifting. The ends of the mold bore I8 are provided withenlargements I9 to receive end plates '20 which may be retained inposition by set screws 2l threaded through the mold wall.

The coating composition described above mayy be applied to the innersurface of the mold 'I0 by spraying apparatus 22, such as illustrated atthe left of Fig. 1. The spraying apparatus 22 is mounted upon a carriage23 having wheels 24 running on rails 25 so that the carriage may bemoved toward and away from the adjacent end of the mold I0. The rails 25are in alignment with the mold and may be supported on ties 26 on thefloor I5. Supported on the carriage 23 is a suitable container 21 forthe coating composition. An electric motor 28 mounted over the container21 may be used for driving an agitator in the container and for`operating a pump therein to circulate the coating composition throughsuitable piping 29, 30, in the lance 3l to the spray device 32 on theouter end thereof. The lance 3| is secured to the carriage 23 in such amanner that movement of the carriage back and forth on the rails 25 willreciprocate the spray device 32 from end to end within the bore I8 andparallel to the mold axis. It is preferably mounted in hangers 33 insuch a manner that it may be adjusted in a vertical direction in orderto vary the distance of the Spray head from the vinner surface of agiven mold, or to adjust the spray head for coating molds of differentdiameters. The carriage 23 may be moved toward and away from the moldbymeans of a piston rod 3| having one end connected to the carriage by anarm 35 and having its opposite end connected 4to a piston operating in acylinder 36 supported alongside one of the rails 25.

With a spraying apparatus of this character the coating composition maybe applied to the inner surface of the mold by reciprocating the spraydevice 32 throughout the length of the bore of the heated, rotating moldas many times as may be necessary to build up a coating having thenecessary thickness and a surface roughness of the` character desiredunder the given casting conditions. A coating which has been foundsatisfactory for casting long tubular bodies by the method ofcentrifugal casting employing the non-'retractive type of pour, isillustrated at 3l in Figs. `4 and 5 of the drawings on a scale which edten times and Fig. 'I being magnified twenty times.

After the coating is applied to the inner surface of the mold, the endplates 20 are positioned in the enlargements I9 and secured therein bythe set screws 2l. Before they are inserted the end plates are providedon their inner faces with a suitable coating which may conveniently besimilar to the coating heretofore described. The

charge of molten metal is delivered to one end of the rotating mold froma pouring device Il which can be an ordinary casting ladle, an in-`duction furnace, or simply a receptacle for molteny metal as illustratedin the drawings. The pouring device 4| is provided with a spout I2 whichis so constructed and arranged as to project into 'the end of the mold,`through a central opening 43 in the adiacent end plate 20, when the`charge is poured. The pouring device 4| may be supported on. a carriage4I provided with a set of wheels 45 running on rails 46 so that it maybe readily moved into and out of pouring position. i

When the charge is poured at a suitable rate into the vmold the thinrefractory coating pre'- viously sprayed on the mold and having both acontrolled thermal barrier action and a controlled surface roughness,performs the functions de-v scribed above in detail and modifies thecasting operation accordingly, whereby to make possible the centrifugalcasting in a permanent metal mold of a long tubular body in which themetal is completely fused throughout its wall and which is substantiallyfree from pinholes, and laps and like defects in its outer portion orsurface. While the outer surface of a casting produced in accordancewith this invention substantially conforms to the roughness of the moldcoating so employed in its production, this roughness is notobjectionable in castings employed for many purposes. However, where asmoother outer surface is required it may be readily obtained bymachining the casting to the degree required to remove the roughnessfound in its outer surface in the as cast condition. Y

The manner in which the invention is employed in the particular methodof centrifugal casting involving a retractive type of pour, will beobvious to those skilled in the art, in view of what has been set forthabove, and hence is not specilcally illustrated. r

An apparatus which has been found satisfactory in the-application of a..thin refractory coating to a, metal mold for centrifugal casting inaccordance with the present invention, forms the subject matter 0f apending application for patent by Alfred Boyles, Serial No. 469,751, ledDecember 19, 1942, for Process and apparatus for coating molds.

Wh'at we desire to claim is:

1. In the method of making a tubular metal casting centrifugally in acylindrical metal mold, the improvement which comprises forming on theinner surface of said mold a` refractory, insulating coating adherent tosaid mold and having a multiplicity of spaced protuberances on its innerface, supporting said mold substantially horizontally and rotating itabout its longitudinal axis, and pouring molten metal into said mold,the magnitude and distribution of said protuberances being such that thetraction of said protuberances on the molten metal serves both toaccelerate the pick-up of the molten metal for rotation with the moldand to retard the flow of the molten metal longitudinally of said mold,and the thickness of the coating being such that the thermal barrieraction thereof serves to momentarily arrest the heat flow to the moldfrom the longitu- 'dinally advancing leading edge of the molten v innerface, supporting said moldl substantially metal to maintain such leadingedge at a temperaturerto fuse with the molten metal subsequently flowingthereover, while maintaining the over-all rate of solidiflcation of thecastingsub.. stantially the same as in the bare metal mold.

2. In the method of making tubular metal castings centrifugallyfin a,permanent metal mold, the improvement which comprises heating said mold,spraying the inner surface of said mold with' a liquid suspension of apowdered refractory material and a binder to form incrementally aninsulating coating adherent to said mold-'and having a multiplicity ofspaced protuberances on its horizontally and rotating it about itslongitudinal axis, and pouring/molten metal into. said mold,`

ances being such that the traction of said protuberances on the moltenmetal serves both to accelerate the pick-up of the molten metal'forrotation with themold and to retard the flow of the molten metallongitudinally of said mold, and the thickness of the coating beingsuch' that the thermal barrier action thereof maintains-the leading edgeof the molten metal at a temperature to fuse with the molten metalsubsequently flowing over it, while maintaining the over-all rate ofsolidiflcation of the casting substantially the same as in the baremetal mold.

3. In the method of making tubular metal castings centrifugally in apermanent metal mold, th'e improvement which comprises heating said moldto a temperature of 250 to 800 F., spraying the inner surface of theheated mold with an aqueous suspension of a powdered refractory materialand a binder to form incrementally thereon an ad= herent, insulatingcoating having a multiplicity of spaced protuberances on its inner face,supporting said mold substantially horizontally and rotating it aboutits longitudinal axis, and pouring molten metal into said mold, themagnitude and distribution of said protuberances being such that thetraction of said protuberances on the molten metal serves both vtoaccelerate the pickup of the molten metal for rotation with the mold andto retard the flow of the molten metal longi..

tudinally of said mold, and the thickness of the crating being such thatthe thermal barrier action thereof maintains the leading edge of themolten metal at a temperature to fuse with the molten metal subsequentlyowing over it.

4. In the method of centrifugally casting tubular metal bodies in apermanent metal mold, the improvement which comprises heating the moldtoa temperature within the range of 250 to 800 F., supporting said moldsubstantially horizontally and rotating it about its longitudinal axis,relatively reciprocating a spray device throughout lthe length of thebore of said mold while discharging from said device an aqueous suspen-'sion of a powdered refractory material and a binder to provide the moldwith an adherent, insulating coating comprising a multiplicity ofincrementally formed, basi-connected protuberances, and pouring moltenmetal into said mold, the magnitude and distribution of saidprotuberances being such that the traction of said protuberances on themolten metal serves both to accelerate the pick-up of the molten metalfor rotation with the mold and to retard the ilow of the molten metallongitudinally of said mold, and the thickness of the coating being suchthat the thermal barrier action thereof maintains the leading edge ofthe molten metal at a temperature to fuse withy the molten metalsubsequently flowing over it.

5. In the method of producing tubular metal castings centrifugally in apermanent metal mold, the improvement which comprises applying to saidmold a refractory, insulating coating adherent to said mold and havingan average thickness of from 0.002 inch to 0.080 inch and having a roughexposed surface within the range of No. 24 to No. 240 abrasive cloth orpaper, supporting said mold substantially horizontally and rotating itabout its longitudinal axis, and pouring molten metal into said mold,the magnitude and distribution of said protuberances being such that thetraction of said rough surface on the molten metal servesboth toaccelerate the pick-up of the molten metal for rotation with the moldand 'to retard the flow of the molten metal longitudinally of said mold,and the thickness of the coating being such that the thermal barrieraction of the coating maintains the leading edge of the molten metal ata temperature to fuse with the molten metal subsequently flowing overit.

6. In the method of producing tubular metal castings centrifugally in apermanent metal mold,

` the improvement which comprises providing the mold with a refractory,insulating coating adherent to said mold and having an average thicknessof from 0.002 inch to 0.080 inch and having on its exposed surface amultiplicity of protuberances the average number of which measured inany direction across the surface of the coating is from 10 to 120 perlineal inch, and which protuberances have an average height above thesurroundingv coatingwithin a range of 0.001 inch to 0.050 inch,supporting said mold substantially horizontally and! rotating it aboutits longitudinal axis, and pouring a charge of molten metal into saidmold.

7. In the method of producing tubular metal castings centrifugally in apermanent metal mold into which the molten metal is poured from a spoutrelatively ilxed lengthwise of the mold, the improvement which comprisesproviding the mold with a refractory, insulating coating adherent tosaid mold and having an average thickness of from 0.01 inch to 0.08 inchand having on its exposed surfaceA a multiplicity of protuberances theaverage number of which measured in any direction across the surface ofthe coating is from 10 to 50 per lineal inch, and which protuberanceshave an average height above the surrounding coating within the range offrom 0.005 inch to 0.050 inch, supporting said mold substantiallyhorizontally and rotating it about its longitudinal axis, and pouringmolten metal into said mold.

8. In the method of making tubular metal castings centrifugally in apermanent metal mold. the improvement which comprises heating said mold,spraying the inner surface of said mold with an aqueous suspension ofsilica flour and bentonite to form incrementally an adherent, insulatingcoating having a multiplicity of spaced protuberances on its inner face,supporting said mold substantially horizontally and rotating it aboutits longitudinal axis, and pouring molten metal into said mold, themagnitude and distribution of said protuberances being such that thetraction of said protuberances on the molten metal serves both toaccelerate the pick-up of the molten metal for rotation with the moldand to retard the ow of the molten metal longitudinally of said mold,and the thickness of the coating being such that the thermal barrieraction thereof maintains the leading edge of the molten metal at'atemperature to fuse with the molten metal subsequently flowing over it.

9. In the method of making tubular metal castings centrifugally in apermanent metal mold, the improvement which comprises heating said mold,spraying the inner surface of said mold with an aqueous suspension ofsilica flour and bentonite in the proportions of from 500 to 1000 gramsof silica flour and from 10 to 40 grams of bentonite, per litre ofwater, to form incrementally an adherent, insulating coating having amultiplicity of spaced protuberances on its inner face, supporting saidmold substantially horizontally and rotating it about its longitudinalaxis, and pouring molten metal into said mold, the magnitude anddistribution of said protuberances being such that the traction of saidprotuberances on the molten metal serves both to accelerate the pick-upof the molten metal for rotation with the mold and to retard the flow ofthe molten metal longitudinally of said mold. and the thickness of thecoating being such that the thermal barrier action thereof maintains theleading edge of the molten metal at a temperature to fuse with themolten metal subsequently ilowing over it.

10. The vmethod of preparingfa permanent metal mold for casting tubularmetal bodies centrifugally therein, which comprises heating said mold`and spraying on the inner surface of said mold a liquid suspension ofpowdered refractory material and a binder in atomized condition so thatthe heat of the mold evaporates the liquid upon contact of the sprayedglobules with the mold tc thereby form incrementally on said mold anadherent, refractory, insulating coating having on its exposed surface amultiplicity of4 irregular. rough protuberances, the magnitude anddistribution of said protuberances being such that the aaeaooe tractionof said protuberances on the molten metal serves both to accelerate thepickup of the molten metal for rotation with the mold and to retard theflow of the molten metal longitudinally of the mold, and the thicknessof the coating being such that the thermal barrier action thereofmaintains the leading edge of the molten metal at a temperature to fusewith the molten metal subsequently ilowing over it.

1l. The method of preparing a permanent metal mold for casting tubularmetal bodies centrifugally therein, which comprises heating said mold toa temperature within the range of 250 to 800 F., and spraying on theinner surface of said mold an aqueous suspension of powdered refractorymaterial and a binder in atomized condition so that the heat of the moldevaporates the liquid upon contact oi. the sprayed globules with themold to thereby form incrementally on said mold a refractory, insulatingcoating having an average thickness within the range of 0.002 inch to0.080 inch and having on its exposed surface a multiplicity ofirregular, rough protuberances, the magnitude and distribution of saidprotuberances being such that the traction of said protuberances on themolten metal serves both to accelerate the pickup of the molten metalfor rotationwith the mold and to retard the iiow of the molten metallongitudinally of the mold.

12. 'I'he method of coating the inner surface of a cylindrical metalmold used for making tubular metal castings centrifugally, whichcomprises heating said mold, and reciprocating a spray device throughoutthe length of the bore of the heated mold in a direction parallel to theaxis thereof while discharging from said device a liquid suspension of apowdered refractory material and a binder in atomized condition sov thatthe heat of the mold evaporates the liquid upon contact of the sprayedglobules with the mold` to thereby form incrementally on said mold arefractory, insulating coating having an average thickness within therange of 0.002 inchl to 0.080 inch and having on its exposed surface amultiplicity of irregular, rough protuberances, the magnitude anddistribution of said protuberances being such that the traction of saidprotuberances on the molten metal serves both to accelerate the pickupof the molten metal for rotation with the mold and to retard the flow ofthe molten metal longitudinally of the mold.

13. A permanent metal mold for centrifugal casting, having adherent onits inner surface an insulating coating of powdered refractory materialand a binder, said coating having an average thickness of from 0.002inch to 0.080 inch and having on its exposed surface a multiplicity ofprotuberances, the magnitude and distribution of said protuberancesbeing such that the traction of said protuberances on the molten metalserves both to accelerate the pickup of the molten metal for rotationwith the mold and to retard the ilow of the molten metal longitudinallyof the mold.

14. A permanent metal mold for centrifugal casting, having adherent onits inner surface an insulating coating of powdered refractory materialand a binder, said coating having an'average thickness of from 0.002inch to 0.080 inch and having a rough exposed surface within the rangeof No. 24 to No. 240 abrasive cloth or paper, the

magnitude and distribution of said protuberances being such that thetraction of said protuberances on the molten metal serves both toaccelerate the pickup of the molten metal for rotation with the mold andto retard the flow of the molten metal longitudinally of the mold.

15. A permanent metal mold for centrifugal casting, having anincremental, insulating coating sprayed thereon and adherent thereto,which coating has an average thickness of from 0.002 inch to 0.080 inchand has on its exposed surface a multiplicity of protuberances theaverage number of which measured in any direction across the surface ofthe coating is from 10 to 120 per lineal inch, and which protuberanceshave an average height above the surrounding coating within a range of0.001 inch to 0.050 inch.

16. A permanent metal mold for centrifugal casting, having anincremental, insulating coating sprayed thereon and consisting ofpowdered refractory material and a binder, said coating having anaverage thickness of from 0.002 inch to 0.080 inch, having on itsexposed surface a multiplicity of protuberances for providing tractionon the molten metal, the magnitude and distribution of saidprotuberances being such that the traction of said protuberances on themolten metal serves both to accelerate the pickup of the molten metalfor rotation with the mold and to retard the flow of the molten metallongitudinally of the mold, and the coating being suillciently adherentto withstand the washing away action of -the molten metal and sumcientlyfriable upon solidiiication of the casting to permit the casting to bereadily withdrawn from the mold.

17. A permanent metal mold for centrifugal casting, having anincremental, insulating coating sprayed thereon and consisting ofpowdered refractory material and a binder, said coating having anaverage thickness within the range of 0.002 inch to 0.080 inch andhaving on its exposed surface a, multiplicity of protuberances theaverage number of which measured in any direction across the surface ofthe coating is from 10 to 120 per lineal inch, and which protuberanceshave an' average height above the surrounding coating within a range of0.001 inch to 0.050 inch, said coating also being suflicientiy adherentto withstand the washing away action of the molten metal andsuiilciently friable upon the solidiiication of the casting to permitthe casting to be readily withdrawn from the mold.

ARTHUR E. SCHUH.

ALFRED BOYLES.

